Image output apparatus, such as drop on demand ink jet printers, laser printers and the like, form images on an image medium as a bitmap of picture elements or "pixels". The pixel bitmap represents the images as a planar array binary digit, where a "0" represents a white pixel and a "1" represents a black pixel (or vice versa). It is often the case that the input image to be transmitted to the image output apparatus is of a lower bitmap resolution than the maximum image bitmap resolution the image output apparatus is capable of producing.
Accordingly, it is highly desirable to convert the low resolution input image to a higher resolution output image before outputting the image to the image output apparatus. U.S. Pat. No. 4,975,785 to Kantor discloses a method for resolution conversation whereby an input resolution of 240 pixels per inch by 240 pixels per inch is converted to an output resolution of 480 pixels per inch by 480 pixels per inch. The input image having the input bitmap resolution is converted to an output image having the output bitmap resolution by taking each pixel of area a.sub.i and dividing it into 4 output pixels of area a.sub.o, where a.sub.o =a.sub.i /4. Once the input pixels are converted to output pixels, it is necessary to "smooth" the output image to take advantage of the finer resolution. Kantor uses a smoothing method taught in U.S. Pat. No. 4,437,122 to Walsh et al. Walsh et al. teaches a method for reducing the stair-stepping in a converted image by selectively replacing each pixel of the input resolution image by a particular 3.times.3 pattern of pixels to obtain an output resolution image having a 1:3 resolution conversion.